The present invention relates generally to a timing acquisition or synchronization method using pseudo-noise (PN) sequences. More particularly, the present invention provides a method for timing acquisition that is based on a bounded timing uncertainty.
Timing acquisition and synchronization is a critical element of many modem electronic systems, such as Digital Communications Systems, Radar Systems, and Digital Signal Processing Systems. The subject of receiver synchronization is discussed in most books on digital communications (see, for example, Sklar, B, Digital Communications, 2nd Ed., Chapter 10, Prentice Hall, Upper Saddle River, N.J., 2001), and, indeed, entire books have been written on the subject of receiver synchronization within digital communications systems (Mengali, U, Synchronization Techniques for Digital Receivers, Springer, New York, N.Y., 1997; Meyr, H. et al, Synchronization in Digital Communications, John Wiley & Sons, Hoboken, N.J., 1990), all of the foregoing books being hereby incorporated by reference. In many applications, such as the extremely dense signal environment of a cellular phone system, the very low signal-to-noise-ratio communications channel of a deep space probe, or the very narrowband system employed by a digital modem in most personal computers, highly efficient signal synchronization techniques are desired. The act of synchronization within a receiver essentially involves replicating the timing information contained in the transmitted signal within the receiver, so that the data information modulated or encoded within the transmitted signal can be extracted. Without first obtaining synchronization, no data information can be extracted from the transmitted signal, and the communications link will fail. Hence, timing acquisition is one of the first processes that takes place when a communications link between a receiver and a transmitter is established.
A well-known technique employed widely in the systems mentioned above is the use of PN sequences for timing acquisition and synchronization. Additionally, due to improved clocks and the omnipresence of GPS, accurate time references are widely available at receivers for use in timing acquisition. Thus, when sending a PN sequence to achieve timing acquisition for many different applications that also have access to GPS and/or improved clocks, the timing uncertainty of these systems has been greatly reduced, resulting in an a priori bound on the timing uncertainty. Because of this bounded timing uncertainty, it is possible to employ shortened timing acquisition PN sequences, or to transmit signals having less power, or to trade other system resources in order to reduce the system overhead associated with the timing acquisition process as a result of the bounded timing uncertainty.
Although the improved clocks have improved the timing acquisition process and created a bounded timing uncertainty, the timing acquisition process is still far from ideal. Systems incorporating timing acquisition sequences range from military applications such as Low Probability of Interception (LPI) and spread spectrum communications to commercial networking applications such as WiFi. Any method that makes the timing acquisition process more efficient for a system has a direct impact on improving the overall performance of the system.
Current systems that use timing acquisition techniques require computation of the autocorrelation properties of the PN sequence over the entire length of the PN sequence. This autocorrelation requirement results in a need for the system to employ longer PN sequences. Further, in a spread spectrum application, the timing acquisition process for the spreading sequence requires more time, which limits the range of the system, and requires greater signal power. Additionally, it is not possible to separate the PN sequence detection process from the timing acquisition process in these spread spectrum sequences. PN detection requires a certain length PN sequence for the system to obtain the necessary correlation gain for the application. Also, the main autocorrelation peak of the PN sequence must be reliably distinguished from all of the sidelobe peaks of the PN sequence.
Real world systems that use timing acquisition methods include Wireless Local Area Network (WLAN) technology including I.E.E.E. Standards 802.11(a), (b) and (g). WLAN is a widely used application in which CDMA or spread spectrum techniques use PN sequences to transmit data. Those sequences have unique characteristics that allow synchronization (timing acquisition) via correlation. Advanced forms of these WLAN protocols could benefit from improvement to timing acquisition.
Bluetooth® technology is a short range spread spectrum system for sending audio to headsets or data between closely spaced devices, e.g., a sensor and a laptop computer. It is widely used in “so-called” Bluetooth® headsets for receiving stereo signals wirelessly in a small area (e.g., the inside a car or within a room). Again, advanced forms of the Bluetooth® protocol could benefit from improvements to timing acquisition.
WiMAX is a wide area network protocol similar to the WLAN protocols but for longer distance communications at higher data rates and is designed to compete with cellular technology and last-mile solutions involving fiber. Further, UWB for video transmission uses even more signal spreading in time as well as frequency (up to 500 MHz in bandwidth) than that used by the WLAN and Bluetooth® protocols. UWB operates within the same bands as many other signals and so must be very low in power. Hence synchronization sequences are used in order to synchronize these very low power signals reliably. Again, advanced forms of these protocols could benefit from improved timing acquisition.
All of the previous techniques describe essentially signal synchronization using PN sequences and correlation. PN sequences and correlation can also be used at a higher level within packetized protocols, which are already symbol synchronized. Each frame (or packet) is sent independently in many applications and frame or packet synchronization must be done to find the beginning of each frame or packet. Frame synchronization for packetized systems may also benefit from improved timing acquisition.
Thus, in view of the disadvantages of the current methods and systems, what is needed is a method to reduce the system resources spent in the timing acquisition process.